She replied: "I believed that they believed it was him but also that they could be wrong."

Asked what she was thinking at the time, she said: "I did not have any intelligence telling me that he was not carrying a bomb and I believed that he was the person from the day before."

Asked if she thought her handling of the situation was proportionate, she said: "I do." Miss Dick, 47, was speaking as a defence witness at the trial of the Met, which is accused of a series of failures leading up to the death of Mr de Menezes, who was shot dead at Stockwell Tube station in 2005.

She said she asked members of her team for a percentage of how certain they were that the man they were following was "Nettletip", codename for Osman, but was never given an answer.

But she said she was also told three times by her Scotland Yard surveillance monitor, codenamed "Pat", and twice by her "silver" commander that it was.

Miss Dick told the jury that in a "fast moving situation" there were no 'golden rules' on the level of identification needed and that various factors fed her decision that day.

She told how the actions of Mr de Menezes, coupled with the events of the July 21 attempted bombings the day before, had led her to think he must be stopped.

"I know how quickly these things can change, but it was possible even though they believed it was him they could change their minds. But I believed they believed it was him."

Ronald Thwaites QC, defending the Met, asked Miss Dick what made her give the order to stop Mr de Menezes. She said: "Firstly I believed that the surveillance team believed it was him.

"Secondly, from the behaviours that had been described to me, given that I thought they thought it was him, it very, very well could have been him.

"The behaviours that were described in terms of nervousness, agitation, sending text messages, using the telephone, then getting on and off the bus all added to the picture-of someone potentially intent on causing an explosion."

Miss Dick also described the moment she gave an order to relatively untrained armed intelligence officers to intervene to stop Mr de Menezes from getting on the Tube.

She said: "That was a big decision, I knew that at the time. It was significant because I could have put them at risk and the public at risk."

When asked how long it took her to make the decision, she said "less than a second".

Mr de Menezes, 27, was shot in the head by officers a day after the failed London bomb plot.

The Met is accused under health and safety laws of overseeing a "fundamentally flawed" operation which led to the electrician's death. It denies the charges.